why limit long exposure to 30 sec

Discussion in 'Canon EOS' started by steve santikarn, Aug 27, 2013.

  1. I am just wondering if there is any good reason/s why the long Exposure time on Canon DSLR is limited to 30 seconds? Things are just getting interesting at 30 seconds for me, and of course I have cable release/timer ect. I just wonder if it is because of a technical/economic reason or lack of demands that kept this cap on long exposure time.
  2. Disclaimer: The following is purely a guess/opinion, so take it with a grain of salt.
    I think it's mostly a leftover from the film days. On many film bodies, the B (bulb) setting wasn't a separate setting, but rather what you got if you kept turning the control dial to one notch longer than 30 seconds, so the more long exposure options you have, the longer you have to sit there twiddling the knob if you want B. And in the film days, you could make a fair argument that it was a technical limitation not of the camera itself, but of the film due to reciprocity failure*.
    As we went into the digital world, I think it pretty much just stayed the same. And from my occasional viewing of the forum here, I can't say I can recall there being a loud and persistent clamour for extending this.
    *: For the most part, films don't respond as well to long exposures as you might like. You'd expect that if you double the time, you get double the exposure, but that tends to fail to be true for longer exposures. That's called reciprocity failure, and it affects each film type differently, so you have to look at the spec sheet for the film to find out how much longer you need to exposure the film to compensate. Even worse, for colour films, the different layers often experience it at different rates, so you not only have to adjust your exposure, you also have to use colour compensation filters (and the filtration you need varies according to exposure length). So perhaps they figured that if you want to shoot an exposure long enough that reciprocity failure is going to affect your image, you'll be using B and timing it manually anyway, rather than choosing from a pre-programmed selection of 1/3- or 1/2-stop increments.
  3. thanks, that makes a lot of sense.
  4. A relatively small number of people ever even use the 30 sec. exposure, unless they are doing something like infrared on a camera with the hot mirror still in place. Like many "seems like a good idea" concepts, there isn't much "clamour" for it, as Steve says. ;)
    A digital analog (not homolog, mind) of reciprocity failure and "grain" in film, is that as exposure is lengthened, the temperature of the individual sensors can rise, causing increasing "noise".
  5. If I'm not mistaken, some cameras - perhaps not Canon - may now essentially build a timer into the camera for longer exposure times. If they don't, I think I'll claim a patent on that! ;-)
    It seems like such a simple, obvious idea, no?
  6. depends on your Canon model, but you can use Magic Lantern to get more control on that,
  7. well, with newer models that come with built-in wifi it is a matter of writing an App to control the long exposure times from
    smart phones or iPads.
  8. As far as film cameras go, AFAIK 30s is the longest measured exposure time that any had. The Canon EF was an example. That was clearly because with such a long exposure reciprocity effects would be severe. They usually had Bulb setting as well.
    In the case of digital cameras there has to be a limit somewhere for practical reasons and after all with the availability of the B setting it is easy to measure longer exposures accurately with a watch or one of the remotes with a timer.
  9. William Michael

    William Michael Moderator Staff Member

    I’ve thought about this before – a long time ago with SLR’s.
    My theory is simple – the TTL metering (especially in automatic modes – Av, Tv, P,) would be problematic.
    Reciprocity and etc.
  10. Really long exposures drain DSLR batteries rapidly.
  11. I agree with the increased noise idea. Sensors heat up with time and the increased sensor temperature raises the noise level. While looking to confirm that idea I came across this article which also points out that increased noise occurs during use with live view as the sensor gets hot. I had not thought of that before :


    Any thoughts on that from people who use live view a lot? I imagine this is something that is present but not normally significant but it would be interesting to hear of others experience.
    Certainly most astrophotography sensor units have built-in cooling to try to overcome the noise problem and hot pixel deletion is a standard procedure.
  12. The old Kodak DCS Pro SLR/c allowed extra long exposure times. It is true that the sensor warms up which itself creates noise.
  13. Yes, it's true that long exposures warm the sensor and that this creates noise (and sometimes worse), but 30 seconds is not a limit. Even with a sensor that is relatively susceptible to noise and high ambient temperatures, you can get nice images at exposures far longer than that. That is why Canon cameras provide long-exposure (subtractive) noise reduction, which lessens pattern noise in exposures longer than one minute. They wouldn't provide that feature of the 30 second limit were designed to prevent longer exposures.
    For example, this was shot with a 50D (not known for low noise) on a hot, cloudy summer night, with an exposure of 10 minutes. I used long-exposure noise reduction, but if I recall, I used only a modest amount of regular NR in lightroom. The yellow light is reflections on the clouds from a few sodium vapor lamps about 7 miles away.

  14. "In the case of digital cameras there has to be a limit somewhere for practical reasons..."

  15. This is a pet peeve of mine as well. I occasionally do night photography and 30 sec. is often too short. I ended up getting an interval timer that not only allows me to take exposures longer than 30sec., but also will keep shooting for what ever duration I like. So I can set up my camera and it'll keep taking images while I go indoors. I really appreciate the gadget when the mosquitoes are active.
  16. As an interesting historical note, one Canon EOS film camera, the Canon EOS 10s, had a built-in interval timer for intervals from 1 sec. to 23 hr. 59 min. 59 sec. for frames 2 to 36.
    Otherwise, for long individual exposures the 10s used a bulb setting with very low drain on the battery.
  17. I can't see any reason for this limitation either. I regularly go beyond the 30 sec limit with my DSLRs with no ill effects. It doesn't matter that noise increases, if that's the only way to get the shot.
    On a side note my old Minox GTE takes automatic exposures of 5 mins and more in moonlight that mostly turn out fine.
  18. William Michael

    William Michael Moderator Staff Member

    "I can't see any reason for this limitation either. I regularly go beyond the 30 sec limit with my DSLRs with no ill effects."​
    . . . the question is not about any limitation of using an exposure time greater than 30s and what ill effects doing so might or might not create.
    The question is about: why 30s is the longest shutter speed that is included in the User Selected Shutter Speeds and also the Automatic Functionality of the camera.
  19. "The question is about: why 30s is the longest shutter speed that is included in the User Selected Shutter Speeds and also theAutomatic Functionality of the camera."
    That's your interpretation of it, but does it matter? The limitation is annoying either way.
  20. William Michael

    William Michael Moderator Staff Member

    I don't believe that's an 'interpretation' of the question at hand:
    1. clearly Canon cameras can make an exposure of longer than 30 seconds
    2. clearly that exposure time is not available as a user selected shutter speed
    3. clearly that exposure time is not available as a automatically selected shutter speed.
    therefore the question clearly is why is such, so?
    Yes it does matter to understand exactly what the question is; if one really wants to look at the possible/probable answers.
    Yes the limitation might be annoying. That is a comment, I think: and whilst conversational, it is not a plausible answer to the question, but it could be a possible answer to the question . . . that is, if Canon's aim is just to annoy their customers!
  21. "In the case of digital cameras there has to be a limit somewhere for practical reasons..."
    The reasons are to do with the writing of firmware for the camera; remember that these cameras are digital devices controlled by firmware; they are no longer purely analogue devices even though the meter is an analogue device.
    For the automatic and semi-automatic modes for which the firmware is written, the maximum settable exposure time has to be scaled to the range of apertures available, the range of ISO settings available and the maximum sensitivity of the meter. Once two parameters are chosen, say ISO and aperture, and the meter is activated, the firmware has to select the other parameter. Whether that choice is made from a look-up table or some other computing method, there has to be a limited number of choices available and thus the manufacturer has to think of a reasonable number for the maximum settable exposure time. Most of them have chosen the longest time generally available on their last film cameras.
    Of course, for manual exposure this limitation does not apply, but the manufacturer's thinking is no doubt that allowing different maximum settable exposure times for manual on the one hand and (semi) automatic on the other could lead to confusion on the part of users.
    And the manufacturer's answer to the question would certainly be "well if you are contemplating using very long exposure times you clearly have the skill to use B and a remote timer or a watch".
  22. Charles, all they would have to do is add a simple timer control on the manual mode or a similar mode. It isn't anything that is beyond the capability of the software or the software engineers.
    We night photographers would be very happy to see such a thing. And, yes, we do "have the skill" to time manually. We also have the skill to focus manually, and select aperture and shutter speed manually, perhaps using a handheld meter. We don't really need IS. We can change lenses, so we don't really need zooms... ;-)
    I'm not sure I get your point...
  23. Sorry I was so long winded. My essencial point was that, as there is a strictly limited range of apertures available and a limited range of ISO chosen by the manufacturer, for a practical auto-exposure system, the available range of exposure times must be limited in such a manner as to relate them to the possibilities allowed by those other parameters. I assume that the manufacturer would want to limit the exposure times allowed in manual mode to the same range as in the automatic modes.

    However, it occurs to me that for manual mode only they could enable an extension of the allowed exposure times via a menu choice just like the extension, upwards or downwards, of ISO possible with some DSLRs. That I think is effectively what you are suggesting.
  24. Can anyone advise me on camera settings, techniques & additional equipment needed to take long exposures of 10+ minutes using a Canon EOS 20D?
    Many thanks
  25. William Michael

    William Michael Moderator Staff Member

    Can anyone advise me on camera settings, techniques & additional equipment needed to take long exposures of 10+ minutes using a Canon EOS 20D?​
    EOS 20D (and your lens)
    Lens Hood
    Preferably no Filters unless necessary for the exposure (as one example - an ND Filter)
    TC-80N3 Timer Remote Controller (also third party manufacturers make copies and also copies with no programming function)
    (Canon also makes a remote only, that is with NO timer –the RS-80N3)
    Solid Tripod and Head Assembly (assumed that you want static images)
    Maybe weight-bags / centre weight for tripod, if vibration is a problem
    Viewfinder Cap
    Major Camera Settings:
    Camera Mode: MANUAL
    Shutter Speed - BULB
    ISO - to suit exposure: obviously lower is usually better.
    Drive: Single Shot
    Aperture - to suit exposure: If possible a mid-range to small aperture will typically suit capturing an image with - less flare; veiling flare and ghost images)
    Inage Capture: raw or raw + JPEG (L)
    Long Exposure Noise Reduction: ON (Custom Function - 02 – select ’1’) Be aware this will double your exposure time, but if you don’t have another method of ‘sophisticated’ NR, in post production it would be a good idea to use this feature – I find it works pretty good.

    Assemble gear and ensure stability.
    Determining the most critical element of the exposure parameters: for example if the (‘a very long’) 10 minute exposure is required to remove the images of people walking through the scene, then that would be the critical element. Therefore, as an example IF we choose to make an EXPOSURE BRACKET, we would probably choose to make it an APERTURE Bracket OR APERTURE / ISO Bracket.
    Determine Exposure and choose if you will shoot BRACKETS or not.
    Frame and focus the scene.
    Select the “Long Exposure” function on the TC-80N3 and input the exposure time required.
    Put the Viewfinder Cap in place: being careful not to bump the framing of the scene.
    Release the shutter . . . and then wait, 10 minutes is probably time enough for one low alcohol beer or a cup of coffee.
    Other stuff:
    “Mirror Up” (also a Custom Function) is not really required – but if you do use it remember that you have the function turned on – because if you forget you might waste a lot of time ‘thinking’ that you have made a long exposure, when all you have done is slap the mirror up.
    Bumping the lens’s Focus Turret after focus has been made is common error – be careful of that. For some lenses, if you have set up the shot and made focus, but then need to wait for the time of day/night to be correct before you release the shutter, you can use the lens hood reverse mounted and that covers the Focus Turret, acting sort of like a shield, so you can’t inadvertently bump it and stuff up your focus setting. If you do this be careful removing and resetting the lens hood.
    The TC-80N3 had a physical Locking Mechanism on the Shutter Release button (i.e. it holds the shutter release “depressed”). You can use that and manually time the exposure with a tine piece if you wish, rather than setting the time for the exposure in the program of the TC-80N3.
    The RS-80N3 has a similar physical locking system on the Shutter Release, so if budget is a concern and the exposure timer function is not a consideration the RS-80N3 would be an option – but be aware that the TC-80N3 provides you with functionalities, such as interval timed exposures.
    The above images are not advocating that you necessary use a Video / Panning head.
    I just often use that head, for these shots, (and not a quick release head).
    Because my panning head has a big base plate and I screw the base plate directly to the camera (or in the case above to the Battery Grip).
    Also I mount the camera sideways, so it covers the length of the head’s base plate, giving good stability and rigidly, to the tripod assembly.

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